Since their unveiling in 1992, mesoporous ceramics have inspired substantial interest, especially by adding self-assembling monolayer compounds to the surface(s) of the mesopores. By varying the terminal group of the self-assembling monolayer, various chemically functionalized materials have been prepared. A mesoporous material is defined as a material, usually catalytic material, having pores with a diameter or width range of 2 nanometers to 0.05 micrometers.
Exemplary of use of self-assembling monolayer(s) on a mesoporous material is the International Application Publication WO 98/34723 (E-1479 CIP PCT). The self-assembling monolayer(s) is made up of a plurality of assembly molecules each having an attaching group. For attaching to silica, the attaching group may include a silicon atom with as many as four attachment sites, for example; siloxanes, silazanes, and chlorosilanes. Alternative attaching groups include metal phosphate, hydroxamic acid, carboxylate, thiol, amine and combinations thereof for attaching to a metal oxide; thiol, amine, and combinations thereof for attaching to a metal; and chlorosilane for attaching to a polymer. A carbon chain spacer or linker extends from the attaching group and has a functional group attached to the end opposite the attaching group.
Methods of attaching and constructing the self-assembling monolayer on a mesoporous material involve solution deposition chemistry in the presence of water. More specifically, as reported by Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. Science, 1997, 276, 923-926 (Feng et al, 1997); and Liu, J.; Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Gong, M. Adv. Mat. 1998, 10, 161-165 (Liu et al., 1998), a synthetic protocol to prepare monolayers of MPTMS (mercaptopropyl trimethoxysilane) within the pores of MCM-41 involved a 1-hour hydration step, followed by a 6-hour silanation step in refluxing toluene. At this stage, the silane coverage is limited to approximately 3.6-4.0 silane molecules/nm2 (this surface density is not enhanced by either extending the reaction time or increasing silane concentration). Following the silanation with a 2-3 hour azeotropic distillation drives the equilibria through the removal of reaction by-products, and increases this surface density to 5.0-5.2 silanes/nm2. This surface density is representative of typical silane-based monolayers. The monolayer coated mesoporous product is then isolated by filtration, washed extensively and then dried for several days. In summary, the overall procedure takes about 10 hours of laboratory prep time and 1-10 days of drying time. The time is driven by the kinetics of getting the self-assembling molecules into the mesopores and getting the water and any other solvent out of the mesopores.
The product obtained exhibits a maximum of 40% of the monolayer silicon atoms fully crosslinked for maximizing monlayer stability. Ideally, 100% of the silicon atoms would be fully crosslinked. Full crosslinking is having three of the four bonding sites linked to another silicon atom via an oxygen atom, with the fourth linked to the functional group terminated hydrocarbon chain. However, the presence of “dangling” hydroxyl groups (OH—) cannot be avoided in the solution method and it is the presence of the “dangling” hydroxyl groups that interferes with the crosslinking of the monolayer, thus placing a practical upper limit on the number of silicon atoms that are fully crosslinked of 40%.
Thermal “curing” of silane monolayers, wherein typical thermal curing (ca. 150° C.), of a silane monolayer creates a terminal to internal silane ratio of 1:2 corresponding to about 60% to 65% of attaching molecules (silicon) fully crosslinked.
Hence, there remains a need for a mesoporous material having self-assembling monolayer thereon with a greater fraction of the assembly atoms fully crosslinked. There is also a need for greater surface density of silicon atoms, which may also be expressed as a greater surface density of monolayer coverage. Finally, there is a need for a method of making these materials that is less time consuming.
The main difficulty in functionalizing microporous materials may be attributed to diffusion of the organic molecules into the small pore channels. In the last few years, both post-silanizatlon and in-situ deposition have been successfully applied to mesoporous materials, in which the pore diameter is usually larger than 2 nm. The mesoporous materials (usually synthesized using surfactant micelles as templates) have very uniform pore sizes. Because of their high surface area and the open pore channels; functionalized mesoporous materials have been investigated for many adsorption and catalysis applications. However due to the large pore size and the amorphous nature of the materials, these materials are not likely to find application as size selective catalysts.
A zeolite is any one of a family of hydrous aluminum silicate minerals, whose molecules enclose cations of sodium, potassium, calcium, strontium, or barium, or a corresponding synthetic compound, used chiefly as molecular filters and ion-exchange agents. Compared to the mesoporous materials, the diffusion of organic molecules in zeolites is severely hindered by the small pore size. Deposition of silanes on the exterior surface is therefore greatly favored over silanation of internal surfaces. Heretofore, it had been believed that introducing organic functional groups to the internal pore surfaces of commercial zeolites to produce size selective microporous catalysts could not be achieved due to the size of the pores.